DE69516667T2 - METHOD AND DEVICE FOR USE OF STEEL SLAG IN CEMENT PRODUCTION - Google Patents

Abstract

A process and apparatus by which steel slag can be added to the feedstock materials fed into the feed-end of a rotary cement kiln to form cement clinkers. The steel slag is crushed and screened to provide steel slag particles having a maximum diameter of 2''.

Description

This invention relates generally to the production of cement clinker in elongated rotary kilns. More particularly, the invention relates to the method and apparatus for the production of cement clinker in conventional elongated wet or dry rotary kilns wherein steel slag is added at the feed end of the kiln with a stream of feed material containing lime so that as the stream of feed material and steel slag moves toward the heat at the heat end of the kiln, the steel slag is melted and diffuses into the feed material to form cement clinker.

State of the art

As mentioned in US 5,156,676, the literature is replete with processes that can enable the calcination and slagging of cement constituents. The typical process using a rotary kiln, either wet or dry, is well known. Cement raw materials such as limestone, clay and sand or the like are finely ground and intimately mixed to provide a substantially homogeneous mixture at the feed or charging end of the kiln. The kiln is inclined downward at an angle such that the heating end of the kiln is below the charging end. The kiln generally has four operating zones including a precalcination zone, a calcination zone, a slagging zone and a cooling zone. Conventional fuel is combined with preheated air and injected into the kiln at the heating end. Fuels such as e.g. B. Natural gas, oil or powdered coal are conventionally used in cement production processes.

As the finely divided cement feedstocks enter the rotary kiln at its feed end, the materials are heated from near ambient temperature to about 538ºC (1000ºF) in the pre-calcination zone. In this zone, the heat of the combustion gases from the calcination zone is used to raise the temperature of the feedstocks. In addition, the kiln may incorporate chain systems or the be attached to the interior of the furnace and used to improve the efficiency of heat exchange between the gases and starting materials.

The temperature of the feedstocks is increased from about 538ºC (1000ºF) to about 1093ºC (2000ºF) as they pass through the calcination zone, and in this zone CaCO₃ is decomposed to form CO₂.

Calcined material at the temperature of about 1093ºC (2000ºF) then enters the slagging or burning zone where the temperature is raised to about 1500ºC (2732ºF). In this zone, the primary raw materials are converted into the typical cement compounds such as tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite. The cement clinkers then leave the slagging zone where the clinkers are cooled and then further processed such as by grinding.

Further, the use of ground blast furnace slag as a cementitious material dates back to 1774. In the production of iron, the blast furnace is continuously fed from above with iron oxide sources, aggregate and fuel. Two products are obtained from the furnace: molten iron, which collects in the bottom of the furnace, and liquid iron blast furnace slag, which floats on top of the molten iron. Both are periodically tapped from the furnace at a temperature of about 1500ºC (2732ºF). The slag consists primarily of silica and alumina, combined with calcium and magnesium oxides from the aggregate. Cementitious activity of this slag for use in mortar or concrete is determined by its composition and the rate at which the molten material is cooled as it emerges from the furnace.

Further, a similar process occurs in the production of steel, with liquid steel slag floating on top of the molten steel. Again, the steel slag consists primarily of silica and alumina, combined with calcium and magnesium oxides. The removal of both steel slag and blast furnace slag is a major disposal problem for their manufacturers.

Both steel slag and blast furnace slag are composed of particles that are very hard. The steel slag particles have sufficient hardness to form glass. cut. The blast furnace slag has always been in the form of a fine powder when used, which means that a large amount of energy must be used to grind and pulverize the steel slag into the finely powdered form. Such a process is disclosed in US 2,600,515, in which a blast furnace slag, in a finely powdered mixture with limestone, is introduced into cement rotary kilns and is immediately introduced into the flame of the furnace. The slag powder is blown in simultaneously and through the same channels as the fuel, namely pulverized coal, heavy oil or gas. This process has some disadvantages. One of the most significant disadvantages is that enormous amounts of energy are required to pulverize and dry the material so that it can be blown into the furnace.

Many of the chemical compounds in steel slag and blast furnace slag are similar to chemical cement compounds and their heat of formation is already controlled in their respective processes. X-ray structure analysis of steel slag shows that the composition is highly fluxed beta-(β)-dicalcium silicate 2CaO.SiO₂ (C₂S). This compound can be converted to 3CaO.SiO₂ (C₂S) with the addition of CaO in the combustion zone of the rotary kiln.

Experience has shown that steel slag has no deleterious effect on the operation of a rotary cement kiln. The emission of volatile materials from the rotary kiln is improved because the slag has been previously heat treated and most of the volatile materials have been removed, i.e. carbon dioxide, carbon, volatile organic substances and the like. However, as mentioned, grinding or pulverizing of the slag is required, thus adding a costly step to the cement manufacturing process.

Disclosure of the invention

Since it has long been known that many of the chemical compounds in steel slag are similar to cement chemical compounds, and since steel slag is available in large quantities and represents a major disposal problem, it would be advantageous to be able to use the steel slag in the cement manufacturing process when used in a much coarser grained state than the pulverized state now required. and if it could be added to the feed material being added to the furnace at the charging end of the furnace instead of its heating end.

The present invention provides such use of steel slag and provides a method and apparatus for using numerous process steel slags which have been crushed and screened to provide a coarse grained condition with a component diameter of up to 51 mm (2"), the coarse grained slag being introduced with the feed material into the feed end of the furnace, thereby obtaining all the advantages of using steel slag without the disadvantage of requiring the provision of fine grinding, pulverizing or fine comminution of the slag.

As previously mentioned, experience has shown that steel slag has no deleterious effect on the operation of a rotary cement kiln. The emission of volatile materials from the rotary kiln is improved because the steel slag has been previously heat treated and most of the volatile materials have been removed, i.e. carbon dioxide, carbon, volatile organics and the like. Due to the previous history of the steel slag, the required steel slag chemistry has already been achieved during the steelmaking process, thus saving energy in the cementmaking process. Thus, a number of advantages exist in using this slag. As previously mentioned, no grinding, pulverizing or comminuting of the slag is initially required. Large quantities of coarse slag (defined herein as steel slag with a component diameter of up to 51 mm (2")) can be incorporated into the cement clinker composition with only minor chemical changes to the regular rotary kiln feedstock. Crushing and screening is only required for slag particles with a diameter greater than 51 mm (2").

Second, no drying of the slag is required. Inherent moisture is normally 1% to 6%. In the wet process rotary kiln system, significant moisture reduction and savings are realized. In the dry process rotary kiln system, drying of the steel slag is not required.

Thirdly, no blockage of the kiln due to sludge ring or clinker build-up was observed. In both the wet and dry process rotary kilns, the Coarse-grained steel slag has a cleaning effect on material build-up as it moves through the furnace.

Fourth, the coarse steel slag can be used as part of the initial feed material and is introduced into the furnace at its feed end. The steel slag and the wet feed material can be injected into the feed end of the rotary kiln as separate materials and can be injected together at the feed end of the furnace without prior mixing.

Fifth, only slight chemical changes in the feed composition are necessary for the normal feed to accommodate the steel slag. This normally means that the feed must be richer in limestone content.

Sixth, the coarse-grained chemical steel slag compound structure transforms the desired cement clinker structure during heat treatment within the rotary kiln by diffusion.

Seventh, significant energy savings are achieved when steel slag is used due to the low temperature at which the steel slag melts and no grinding or pulverizing of the steel slag is required.

Eighth, production increases are mostly proportional to the amount of steel slag used.

Ninth, the environmental condition of the rotary kiln process improves due to the low volatile content of the steel slag.

Tenth, steel slag recycling improves the environment because it provides an important use for the large amounts of available steel slag and disposal of steel slag is currently an environmental problem.

Eleventh, the cost of cement production is considerably reduced due to energy savings and the abundant supply of cheap steel slag. Thus, it is an object of the present invention to provide an improved method and apparatus for To provide operation of a rotary kiln for the production of cement clinker using coarse-grained steel slag, a by-product of steelmaking processes.

It is a further object of the present invention to introduce the coarse steel slag into a cement manufacturing rotary kiln at its feed end.

It is yet another object of the present invention to utilize coarse steel slag having various particle sizes from a maximum value of substantially 51 mm (2") in diameter and below.

Thus, the present invention relates to a method of producing cement clinker using an elongated rotary cement kiln having a feed end and a heat end, the heat end being inclined downwardly with respect to the feed end, the method comprising the steps of directing heat from a heat source into the heat end of the kiln, introducing a stream of feed material containing lime into the feed end of the kiln so that the stream of feed material moves towards the heat at the heat end of the kiln, and adding a predetermined amount of crushed and screened steel slag to the stream of feed material at the feed end of the kiln so that as the stream of feed material and steel slag moves towards the heat end of the kiln, the steel slag is melted by the heat and diffuses into the feed material. to form cement clinker.

The invention also relates to an apparatus for forming cement clinker comprising a rotary cement kiln having a feed end and a heating end, the heating end being inclined downwardly with respect to the feed end, and with a heat source at the heating end for heating the interior of the rotary kiln, and a conveying means for introducing a stream of feed material containing lime and steel slag into the feed end of the rotary kiln, so that as the stream of feed material and steel slag moves towards the heating end of the kiln, the steel slag diffuses into the feed material by the heat to form cement clinker.

Short description of the drawings

These and other more detailed objects of the present invention are more fully disclosed in the following detailed description of the drawings, in which:

Fig. 1 is a basic diagrammatic representation of a rotary kiln system of the present invention for forming cement clinker in which the feed material and steel slag are introduced together into the feed end of the rotary kilns;

Fig. 2 is a diagrammatic representation of the feed material and the steel slag which are fed separately into the feed end of the rotary kiln;

Fig. 3 is a flow diagram of the process in which the feed material and steel slag are introduced into the feed end of the furnace in a combined mixture; and

Fig. 4 is a flow diagram of an alternative process in which the feed material and the steel slag are introduced separately into the feed or feed end of the rotary kiln.

Best mode for carrying out the invention

The present invention enables steel slag to be added to the furnace feed material as a separate component at the feed end of the cement rotary kiln in various particle sizes up to a maximum diameter of 51 mm (2"). For the most part, steel slag has particles with a diameter below 51 mm (2") and thus a crushing and screening process is required to achieve only the desired maximum particle size. No grinding, pulverizing or fine grinding of steel slag is required in the present invention. The invention provides a method of using various steel slags in a much coarser grained state than previously known in cement rotary kiln processes which enables the elements in the chemical compounds of the steel slag, i.e. C₂S and the like, become an integral part of the cement clinker. As will be understood by those skilled in the art, the chemistry of the slag must be understood and controlled as part of the total constituents of the cement and thus the amount of steel slag added to the feedstock must be balanced with the feedstock materials and their chemical compounds.

In a laboratory furnace firing test of 100% slag, the melting point of steel slag was determined and is the key to its use in a cement kiln. As can be seen from Table I, the melting point was determined to be 1000ºC (2372ºF), which allows the addition of slag to the feed end of the furnace with fairly large particle sizes of up to 51 mm (2") diameter. Table I Steel Slag Laboratory Furnace Firing

The tests shown in Table I were run for 15 minutes at each temperature with a slag size of about 3/8" particles. As a result of these tests, it was determined that the slag does not thicken a slurry in the chain section of the rotary kiln, does not cause sludge rings, or increases dust loss due to particle size. It also reduces the moisture content to about 2.2%. The steel slag begins to melt and combine with other feedstocks somewhere between the calcination zone and the combustion zone in the rotary kiln. Because of the low melting point, it is not necessary to finely grind, pulverize, or crush this material as in the prior art, which requires that 80% of the material through a 75 mm (200-mesh) sieve allows for chemical combination with other constituents. C₂S formation is already complete in the steel slag and C₃S formation takes place in the rotary kiln in the same temperature zone where it melts. X-ray structure analysis of the steel slag shows that the composition is a highly fluxed beta (β) dicalcium silicate 2 CaO.SiO₂ (C₂S). This compound can be converted to 3CaO.SiO₂ (C₃S) with the addition of CaO in the firing zone of the rotary kiln. C₃S is the main strength-bearing compound in cement.

The apparatus of the present invention is illustrated in Figure 1. The apparatus 10 includes the rotary kiln 12, supported in a well known manner by flanges 14 which rotate with the kiln. The kiln has a feed end 16 and a heating end or combustion zone 18. The heating end 18 is inclined downwardly with respect to the feed end 16, as is well known in the art. A fuel source 20 creates a flame 22 in the heating end 18 of the rotary kiln 12 to provide a temperature of about 1500°C (2732°F). Cement raw materials or feed material, such as limestone, clay, sand or the like, is brought to the rotary kiln 12 by a variable speed conveyor belt 24. If a wet slurry is used, the variable speed conveyor 24 will carry the feed material to a mill 26 and from the mill 26 to the feed end 16 of the rotary kiln 12. The feed material moves in a stream 28 through the rotating kiln 12 toward the flame 22. The well known chemical processes take place within the kiln 12 and the cement clinker 30 exits the hot end 18 of the kiln 12 for further processing. Pollution control devices 32 and 34, well known in the art, are provided at the hot end and feed end of the kiln 12, respectively. At the hot end 18, outside the pollution control device 32, exhaust gases 38 are exhausted to the atmosphere and regenerated waste products 40 are recovered.

At the feed end 16, the pollution control device 34 removes the exhaust gases 36 which are emitted and regenerates the waste products at 42.

In the present invention, the steel slag 44 is transported by a conveyor 46, such as a variable speed conveyor belt, to the feed material 48 which is discharged through a dust hopper 56 (Fig. 2) at the feed end 16 of the rotary kiln 12. A controller 25 controls the speed of the conveyor belts 24 and 46 so that the correct proportion of steel slag 44 is provided relative to the feed material, depending on its chemical compositions. Such control is well known in the art and will not be discussed in detail.

Fig. 2 is a diagrammatic representation of the apparatus for providing a separate feed of steel slag and feed material into the feed end of the rotary kiln 12. In Fig. 2 it can be seen that the steel slag 50 is dropped into a hopper 52 and transported upwardly by a conveyor system 54 where it is deposited at 55 so as to pass through the dust hopper 56 to the feed end 16 of the rotary kiln 12. The feeding of the material to the feed end of the kiln can be accomplished in any well known manner. In a similar manner, the feed material 58 is dropped into a hopper 60 where it is transported upwardly by conveyor means 62 and dropped into the hopper 56 at 64 for introduction into the feed end 16 of the rotary kiln 12. Both the apparatus of Fig. 1 and Fig. 2 provide the desired results.

Table II presents the results of chemical analysis of six samples of steel slag taken from a steel slag stock. Of course, the chemical analysis of the steel slag may vary from the values in Table II depending on the slag. Table II Steel slag

It can be seen that the steel slag compositions are very uniform and suitable for the manufacture of cement. It can also be seen that the average free lime is 0.5% and that the average LOI (loss on ignition) is 1.4. The free moisture is 1% and the bound moisture is 1%.

X-ray diffraction analysis of the steel slag shows that the composition is highly fluxed beta-(β)-dicalcium silicate 2CaO.SiO₂ (C₂S). This compound can be converted to 3CaO.2SiO₂ (C₃S) in the combustion zone using additional CaO.

The reaction is 2CaO.SiO₂ + CaO + heat → 3CaO.SiO₂. C₃S is the main strength compound in cement.

Table III illustrates the typical blend calculations for a feedstock containing 0% steel slag, 89.67% limestone, 4.42% shale, 4.92% sand and 0.99% shale. Table III Type I LA Blend Calculation - 0%Slag

Table IV illustrates the blend calculations for a feedstock of 90.79% limestone, 3.64% shale, 5.36% sand and 0.21% ore with the addition of 5% slag, and Table V illustrates the blend calculations for a feedstock of 91.43% limestone, 2.75% shale, 5.82% sand and 0% ore with the addition of 10% slag. Table IV Type I LA Blend Calculation - 5% Slag Table V Type I LA-mix calculation-10% slag

From Tables III, IV and V it can be clearly seen that the addition of slag is suitable as a starting material for the production of cement clinker.

Fig. 3 illustrates the process of the present invention wherein the feed material and steel slag as illustrated in Fig. 1 are combined prior to entering the furnace at the charging end thereof. At step 76, the feed material is provided and combined at step 78 with the steel slag which has been crushed and screened to a maximum diameter of 51 mm (2 inches) at step 80. The combined The selected material is then introduced into the feed end of the rotary kiln at step 82.

In Fig. 4, the process feeds the steel slag and the feed material into the feed end of the rotary kiln separately, as illustrated in Fig. 2. In such a case, at step 66, the feed material is provided and conveyed by a conveyor to the inlet or feed end of the rotary kiln at step 68. The steel slag is ground and screened to obtain a maximum diameter of 51 mm (2") at step 72 and the resulting final product is conveyed to the inlet or feed end of the rotary kiln at step 74. At step 70, the feed material and steel slag are heated in the rotary kiln until cement clinker is formed.

Thus, a method and apparatus for forming cement clinker is disclosed with the addition of coarse steel slag which is introduced with the feed material into the feed end of the rotary kiln. Coarse steel slag is defined herein as steel slag which has been crushed and screened to have a maximum diameter of 2". Many advantages are obtained by the present invention. No grinding, pulverizing or refining of the slag is necessary. Large amounts of coarse slag up to a particle size of 2" can be incorporated into the cement clinker composition with only minor chemical changes required in the regular material feed to the rotary kiln.

No drying of the slag is required. The inherent moisture is typically one to six percent. In the wet process rotary kiln system, significant moisture reduction and savings are realized. In the dry process rotary kiln system, the steel slag can be dried, but this is not necessary.

With the present invention, coarse steel slag can be used in the production of cement clinker via the rotary kiln as part of the initial feed material. The steel slag and wet (or dry) feed material are injected into the feed end of the rotary kiln as separate materials. They can also be injected together after prior mixing at the feed entrance of the kiln. No clogging of the kiln was observed due to sludge. ring or clinker build-up. In both wet and dry process rotary kilns, the steel slag has a cleaning effect on material build-up as it moves through the kiln.

Only slight chemical changes are required for the normal feedstock to accommodate the steel slag. This normally means that the feedstock must have a richer lime content. The chemical compound structure of the coarse steel slag transforms to the desired cement clinker structure during heat treatment within the rotary kiln by diffusion. Since grinding, pulverizing or comminuting of the steel slag is not required, significant energy savings are realized using this invention to produce cement clinker. Production increases are mostly proportional to the amount of slag used. Moreover, the environmental condition of the rotary kiln process improves due to the low volatile content of the steel slag. Moreover, recycling steel slag improves the environment and provides a useful market for steel slag rather than requiring vast areas of land for storage. Thus, recycling steel slag improves the environment and significantly reduces the cost of cement production.

While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but, on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the scope of the invention as defined by the appended claims.

Claims (8)

1. A process for producing cement clinker using an elongated rotary cement kiln having a feed end and a heating end, comprising the steps of: Directing heat from a heat source into said heat end of the furnace; introducing a stream of feed material containing lime into said feed end of the furnace so that the stream of feed material moves towards said heat at the heat end of the furnace; and adding a quantity of steel slag to said stream of feed material at said feed end of the furnace such that as the stream of feed material and steel slag moves toward said heat end, the steel slag is melted by said heat and diffuses into the feed material to form cement clinker, characterized in that the amount of steel slag is crushed and sieved before being introduced into the charging end of the furnace. 2. A method according to claim 1, further including the step of crushing and screening said steel slag to obtain particles having a maximum diameter of substantially 51 mm (2") for addition to said stream of feed material. 3. A method according to claim 1 or 2, wherein the steel slag is added to the feed end of the furnace as a material separate from the feed material. 4. A method according to claim 1 or 2, wherein the steel slag and the feed material are mixed prior to introduction into the feed end of the furnace. 5. A process according to any one of the preceding claims 1-4, which uses a wet process rotary kiln to receive the stream of feed material and steel slag. 6. A process according to any one of the preceding claims 1-4, further including the step of using a dry process rotary kiln to receive the stream of feed material and steel slag. 7. A process according to any one of the preceding claims 1-6, wherein said steel slag has a chemical composition of 2CaO.SiO₂ (C₂S). 8. An apparatus for carrying out a method according to any one of the preceding claims 1-7, which is provided with an elongated cement rotary kiln having a feed end and a heating end, the heating end being inclined downwards with respect to the feed end, as well as with a heat source for directing heat into the heating end, said apparatus further comprising: means for introducing a stream of feed material containing lime into said feed end of the furnace so that the stream of feed material moves toward said heat at the heat end of the furnace; means for crushing and screening a quantity of steel slag; and means for adding the quantity of steel slag to said stream of feed material at said feed end of the furnace so that as the stream of feed material and steel slag moves toward said heat end, the steel slag is melted by said heat and diffuses into the feed material to form cement clinker.